The present invention relates generally to a vibration-insulating device used for mounting an engine, and more particularly to a fluid-sealed vibration-insulating device for engine mounting purposes in which a dilatant fluid is sealed.
A vibration-insulating device for an engine mount must prevent "idling vibration" from developing when vibrations of an engine in an idling condition are transmitted to a vehicle body, and also must prevent "shaking" from developing when the engine resonates with vibrations of tires during the travel of the vehicle. One example of a conventional vibration-insulating device for preventing such vibrations is a fluid-sealed vibration-insulating device (shown in FIG. 1) in which two liquid chambers, provided within the device, communicate with each other through an orifice. More specifically, this fluid-sealed vibration-insulating device includes a connection member 60 for mounting on an engine, a cup-shaped holder 90 for mounting on a member on a vehicle body, an insulator 50 of vibration-insulating rubber or the like provided between the connection member 60 and the holder 90, a main chamber 20 and an auxiliary chamber 30 (in which a non-compressive fluid is sealed) provided below the insulator 50, an annular orifice 10 communicating the main and auxiliary chambers 20 and 30 with each other, an air chamber 70 into which air is introduced, and a diaphragm 40 separating the air chamber 70 from the auxiliary chamber 30. The vibration of the engine during idling, that is, vibration having a frequency of about 30 Hz and a relatively small amplitude (0.05.about.0.1 mm), is prevented from being transmitted to the vehicle body by a low dynamic spring constant of the insulator 50, thereby preventing the idling vibration. The vibrations of the tires during the travel of the vehicle, that is, vibration having a frequency of about 10 Hz and a relatively medium amplitude (0.1.about.0.5 mm), is prevented from being transmitted to the engine by a damping effect of the orifice, thereby preventing shaking of the vehicle.
In newer automobiles, however, vibration of the engine having a very low frequency of about 5 Hz (which is lower than those of the above idling vibration and shaking) has become a serious problem. The very low-frequency (about 5 Hz) engine vibrations develop, for example, during the cranking of the engine and when abruptly starting and accelerating the automobile, and have a relatively large amplitude (0.5.about.5.0 mm). The above conventional fluid-sealed vibration-insulating device cannot prevent the transmission of such very low-frequency, large-amplitude vibrations.
A vibration-insulating device for preventing the transmission of very low-frequency, relatively large-amplitude vibrations is proposed in U.S. Pat. No. 4,852,533. This vibration-insulating device comprises a plurality of first thin plates fixedly secured to a vibrating member, a plurality of second thin plates each fixedly secured to a support member, and interposed between respective adjacent ones of the first thin plates, and a dilatant fluid filled in a gap between any two adjacent ones of the thin plates. When very low-frequency (about 5 Hz), relatively large-amplitude vibrations are applied to this vibration-insulating device, the dilatant fluid is subjected to a shearing force by the first and second thin plates, and its viscosity (dilatancy) is abruptly increased. This increased viscosity prevents the transmission of the very low-frequency, relatively large-amplitude vibrations. Although this vibration-insulating device can prevent the transmission of very low-frequency (about 5 Hz), relatively large-amplitude vibrations, it cannot sufficiently prevent the transmission of high-frequency (about 30 Hz), relatively small-amplitude vibrations and the transmission of low-frequency (about 10 Hz), relatively medium-amplitude vibrations. Further, this vibration-insulating device is extremely complicated in construction.